This invention relates to semiconductor processing, particularly to a chemical mechanical polishing method and apparatus with which to achieve superior global and local wafer planarity control.
Modem integrated-circuit technology is capable of packing a large number of circuit-components at near the surface of a semiconductor wafer by scaling down the feature size of the circuit-components and connects the components with a large number of metal lines imbedded in a matrix of multiple layers of metal and dielectric material. Beneath the silicon wafer surface, the circuit-elements are isolated from each other by regions of silicon dioxide in shallow trenches to prevent unwanted electrical current passage between the circuit-elements.
Because of the downward scaling of the feature size and the increasing complexity of interconnecting scheme, the wafer process requires a high degree of wafer surface planarization. Specifically, the wafer surface must be planarized locally as well as globally after the formation of the shallow trenches and at all interconnect levels. Currently the technique of chemical mechanical polishing is the only satisfactory technique with which the necessary degree of planarization can be achieved.
In a chemical mechanical polishing operation, a semiconductor wafer is mounted upside down on a wafer-carrier, and the carrier is pressed downward against a polishing pad, which is motion with respect to the rotating wafer-carrier. Slurry comprised of silica or cerium oxide particles, for example, suspended in alkaline to slightly acid solution drips onto the polishing pad that has flow channels machined to transport the slurry beneath the rotating wafer carrier where it polishes the wafer surface.
The most popular type of chemical mechanical polisher is the rotary polisher in which both a platen and the wafer-carrier rotate. The carrier holds the wafer face down, and applies a downward force against the surface of the pad. The pad is mounted on a rotating platen by waterproof adhesive. The wafer-carrier rotates about the center point of the wafer and it oscillates along a radius of the platen such that the entire wafer surface contacts the polishing pad during the polishing operation.
Some types of the polishing pads are made of materials that absorb the slurry; other types are made of materials that do not have the ability to absorb the slurry.
The rate of removal of the target material is a function of the pressure that the pad exerts on the wafer surface, the relative speed between the pad and the wafer surface, and the nature of the slurry.